1,036 research outputs found
Solitonic Phase in Manganites
Whenever a symmetry in the ground state of a system is broken, topological
defects will exist. These defects are essential for understanding phase
transitions in low dimensional systems[1]. Excitingly in some unique condensed
matter systems the defects are also the low energy electric charge excitations.
This is the case of skyrmions in quantum Hall ferromagnets[2] and solitons in
polymers[3]. Orbital order present in several transitions metal compounds[4-6]
could give rise to topological defects. Here we argue that the topological
defects in orbital ordered half doped manganites are orbital solitons.
Surprisingly, these solitons carry a fractional charge of e/2, and
whenever extra charge is added to the system an array of solitons is formed and
an incommensurate solitonic phase occurs. The striking experimental asymmetry
in the phase diagram as electrons or holes are added to half doped
manganites[7-12], is explained by the energy difference between positive and
negative charged solitons. Contrary to existent models that explain coexistence
between phases in manganites as an extrinsic effect[13-14], the presence of
inhomogeneities is naturally explained by the existence of solitonic phases.
The occurrence and relevance of orbital solitons might be a general phenomena
in strongly correlated systems.Comment: 10 pages, 5 figures include
Canted phase in double quantum dots
We perform a Hartree-Fock calculation in order to describe the ground state
of a vertical double quantum dot in the absence of magnetic fields parallel to
the growth direction. Intra- and interdot exchange interactions determine the
singlet or triplet character of the system as the tunneling is tuned. At finite
Zeeman splittings due to in-plane magnetic fields, we observe the continuous
quantum phase transition from ferromagnetic to symmetric phase through a canted
antiferromagnetic state. The latter is obtained even at zero Zeeman energy for
an odd electron number.Comment: 5 pages, 3 figure
Navier-Stokes transport coefficients of -dimensional granular binary mixtures at low density
The Navier-Stokes transport coefficients for binary mixtures of smooth
inelastic hard disks or spheres under gravity are determined from the Boltzmann
kinetic theory by application of the Chapman-Enskog method for states near the
local homogeneous cooling state. It is shown that the Navier-Stokes transport
coefficients are not affected by the presence of gravity. As in the elastic
case, the transport coefficients of the mixture verify a set of coupled linear
integral equations that are approximately solved by using the leading terms in
a Sonine polynomial expansion. The results reported here extend previous
calculations [V. Garz\'o and J. W. Dufty, Phys. Fluids {\bf 14}, 1476 (2002)]
to an arbitrary number of dimensions. To check the accuracy of the
Chapman-Enskog results, the inelastic Boltzmann equation is also numerically
solved by means of the direct simulation Monte Carlo method to evaluate the
diffusion and shear viscosity coefficients for hard disks. The comparison shows
a good agreement over a wide range of values of the coefficients of restitution
and the parameters of the mixture (masses and sizes).Comment: 6 figures, to be published in J. Stat. Phy
Equilibration and symmetry breaking in vibrated granular systems
The steady states of two vibrated granular gases separated by an adiabatic
piston are investigated. The system exhibits a non-equilibrium phase transition
with an spontaneous symmetry breaking. Even if the gases at both sides of the
piston have the same number of particles and are mechanically identical, their
steady volumes and temperatures can be rather different. The transition can be
explained by a simple kinetic theory model expressing mechanical equilibrium
and the energy balance occurring in the system. The model predictions are in
good agreement with molecular dynamics simulation results. The macroscopic
description of the steady states is discussed, as well as some physical
implications of the symmetry breaking.Comment: 5 figure
Disorder-Induced First Order Transition and Curie Temperature Lowering in Ferromagnatic Manganites
We study the effect that size disorder in the cations surrounding manganese
ions has on the magnetic properties of manganites. This disorder is mimic with
a proper distribution of spatially disordered Manganese energies. Both, the
Curie temperature and the order of the transition are strongly affected by
disorder. For moderate disorder the Curie temperature decreases linearly with
the the variance of the distribution of the manganese site energies, and for a
disorder comparable to that present in real materials the transition becomes
first order. Our results provide a theoretical framework to understand disorder
effects on the magnetic behavior of manganites.Comment: 4 pages, three figures include
Transport coefficients for dense hard-disk systems
A study of the transport coefficients of a system of elastic hard disks,
based on the use of Helfand-Einstein expressions is reported. The
self-diffusion, the viscosity, and the heat conductivity are examined with
averaging techniques especially appropriate for the use in event-driven
molecular dynamics algorithms with periodic boundary conditions. The density
and size dependence of the results is analyzed, and comparison with the
predictions from Enskog's theory is carried out. In particular, the behavior of
the transport coefficients in the vicinity of the fluid-solid transition is
investigated and a striking power law divergence of the viscosity in this
region is obtained, while all other examined transport coefficients show a drop
in that density range.Comment: submitted to PR
Understanding the dynamics of fractional edge states with composite fermions
Fractional edge states can be viewed as integer edge states of composite
fermions. We exploit this to discuss the conductance of the fractional
quantized Hall states and the velocity of edge magnetoplasmons.Comment: 3 pages, revte
Diffusion in a Granular Fluid - Theory
Many important properties of granular fluids can be represented by a system
of hard spheres with inelastic collisions. Traditional methods of
nonequilibrium statistical mechanics are effective for analysis and description
of the inelastic case as well. This is illustrated here for diffusion of an
impurity particle in a fluid undergoing homogeneous cooling. An appropriate
scaling of the Liouville equation is described such that the homogeneous
cooling ensemble and associated time correlation functions map to those of a
stationary state. In this form the familiar methods of linear response can be
applied, leading to Green - Kubo and Einstein representations of diffusion in
terms of the velocity and mean square displacement correlation functions. These
correlation functions are evaluated approximately using a cumulant expansion
and from kinetic theory, providing the diffusion coefficient as a function of
the density and the restitution coefficients. Comparisons with results from
molecular dynamics simulation are given in the following companion paper
Granular clustering in a hydrodynamic simulation
We present a numerical simulation of a granular material using hydrodynamic
equations. We show that, in the absence of external forces, such a system
phase-separates into high density and low density regions. We show that this
separation is dependent on the inelasticity of collisions, and comment on the
mechanism for this clustering behavior. Our results are compatible with the
granular clustering seen in experiments and molecular dynamic simulations of
inelastic hard disks.Comment: 4 pages, 5 figure
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